Unstable test Http2Test.testKeepAliveTimeout

### Version

main

### Context

In https://github.com/eclipse-vertx/vert.x/actions/runs/20499367576/job/58904378867 the test failed, and it is an unstable test.

When analyzing the test, I found the following:

* `Http2ClientConnectionImpl.onStreamClosed()` calls `super.onStreamClosed(s)` which will return the connection to the pool
* When doing that, the `expirationTimestamp` is not set yet, and due to that the `isValid()` will return false and evict the `SharedHttpClientConnectionGroup.checkExpired()` will have a short time window to close the connection before the `Http2ClientConnectionImpl.onStreamClosed()` set the `expirationTimestamp` and make the connection valid again.

### Steps to reproduce

I was able to reproduce it locally with several breakpoints and some delays in the test.

I'll prepare a pull request.

### Do you have a reproducer?

_No response_